1. Field of the Invention
The present invention relates to a four-cycle overhead valve engine including intake and exhaust valves mounted atop a cylinder block and push rods arranged in a lateral portion of the cylinder block for driving the intake and exhaust valves.
2. Description of the Prior Art
In the four-cycle overhead valve engine now in use, push rods for driving respective intake and exhaust valves mounted above an engine cylinder block are disposed in a portion of the wall of the engine cylinder block laterally of a cylinder bore in which a piston reciprocates and a breather passage for communicating a crank chamber and a rocker arm chamber together is disposed in another portion of the wall of the engine cylinder block laterally of the cylinder bore remote from the push rods, for example, spaced 180xc2x0 from the push rods with respect to a cylinder axis of the cylinder bore.
In this type of the known four-cycle overhead valve engine, the engine cylinder block tends to be bulky in size, having a lateral wall protrusion for accommodating the breather passage at such another portion of the cylinder wall opposite to the portion where the push rods are accommodated.
Apart from the engine discussed above, another four-cycle overhead valve engine is known, in which the breather passage is defined at a location adjacent a flywheel in a portion of the wall opposite to the portion of the wall of the engine cylinder block where the push rods are accommodated.
In this second mentioned four-cycle overhead valve engine, since the breather passage is separated a distance from a push rod chamber in which the push rods are operatively accommodated, an oil return passage through which an oil accumulated within the breather passage is returned to the push rod chamber is required to have an increased length, resulting in complexity in structure.
In view of the foregoing, the present invention is intended to provide a four-cycle overhead valve engine wherein a breather passage is defined in a manner to avoid an undesirable an increase in size of the engine as a whole and also to avoid an increase in length of the breather passage.
In order to accomplish the foregoing object of the present invention, a four-cycle overhead valve engine according to the present invention includes a cylinder block, and push rods for driving intake and exhaust valves, respectively, that are mounted above the cylinder block. The push rods are operatively arranged in a side wall of the cylinder block, and a breather passage connecting a crank chamber and a rocker arm chamber with each other is made up of a first passage portion defined in the wall of the cylinder block at a location adjacent a flywheel and the push rods, and a second passage portion extending from one side adjacent the push rods in a direction away from the push rods. A check valve for the breather passage is disposed at a junction between the first and second passage portions.
According to the present invention, a lateral wall protrusion of the cylinder block at a location opposite to the push rods can be minimized, the cylinder block can be correspondingly reduced in size to thereby suppress increase of the size of the engine as a whole. Also, since the check valve is disposed in the vicinity of a push rod chamber accommodating the push rods, an oil return passage can have a substantially reduced length, where the oil return passage is employed to return an oil, tending to accumulate in a portion of the breather passage downstream of the check valve, to the push rod chamber. The second passage portion referred to above may be defined beneath, for example, an undersurface of a cylinder head mounted atop the cylinder block.
In one preferred embodiment, the cylinder block has a cylinder bore having a longitudinal axis and is arranged with its longitudinal axis inclined relative to a vertical direction and also to a horizontal direction so as to be diagonally upwardly oriented and wherein a push rod chamber accommodating the push rods in the cylinder block is defined in an underside portion of the wall of the cylinder block, that is positioned diagonally downwardly of a cylinder bore so as to extend along the cylinder bore.
With such a structure, a blow-by gas can easily be circulated to an upper portion of a rocker arm chamber accommodating rocker arms for driving the intake and exhaust valves through the second passage portion of the breather passage. Accordingly, an oil separated from the blow-by gas can smoothly flow onto the rocker arm by the effect of gravity and can then be easily returned to a crank chamber through the rocker arm chamber and the push rod chamber.
The breather passage referred to above may preferably include a third passage portion connecting the second passage portion with the rocker arm chamber. This third passage portion is defined in a portion of the cylinder head on one side remote from the push rods.
In one preferred embodiment of the present invention, the four-cycle overhead valve engine may further include an oil return passage for connecting a portion of the breather passage downstream of the check valve with the push rod chamber.
The oil contained in the blow-by gas tends to accumulate in the portion of the breather passage downstream of the check valve when the check valve is closed. However, according to the present invention, the oil accumulating in the portion of the breather passage downstream of the check valve with respect to the direction of flow of the blow-by gas can be returned to the push rod chamber through the oil return passage and then back to the crank chamber. Also, since the first passage portion of the breather passage is defined in the vicinity of the push rod chamber in the cylinder block and, hence, the junction between the first and second passage portions is located in the vicinity of the push rod chamber, the oil return passage referred to above can have a reduced length. It is to be noted that the oil return passage may be defined in the undersurface of the cylinder head mounted atop the cylinder block.
In one preferred embodiment of the present invention, a gasket may be interposed between the cylinder block and the cylinder head, and the check valve is provided in the gasket. This check valve is preferably employed in the form of a reed valve.
Where the check valve is provided in the gasket sandwiched between the cylinder head and the cylinder block, no extra fixture which would otherwise be needed to install the check valve is necessary, resulting in simplification of the structure.
In one preferred embodiment of the present invention, a stopper is defined in the cylinder head for regulating an opening of the check valve.
Since the stopper for regulating the opening of the check valve is formed integrally with the cylinder head, no element functionally corresponding to the stopper is needed to be formed separately, resulting in simplification of the structure.